The ion assisted deposition of a thin film has been recognized as a means of improving or controlling a variety of thin film properties such as adhesion, density, stress, or even the selection of a preferred form of molecular bonds as in the deposition of diamondlike carbon. Some examples of ion assisted deposition are described in an article by Harper, et al. which appeared in the book Ion Bombardment Modification of Surfaces: Fundamentals and Applications, O. Auciello and R. Kelly, eds., Elsevier Science Publishers B. V., Amsterdam, 1984, beginning on page 127. The range of 1-100 eV/atom was indicated as the range of interest for the modification of thin film properties in the aforementioned article. Note that this range is eV/atom, which equals the total ion energy arriving at the target divided by the number of atoms that have been deposited. The same ion energy can be obtained with either a few ions arriving at high energy or many ions arriving at low energy.
Ion assisted deposition is typically carried out with complicated apparatus that uses separate devices to perform the deposition and ion assist functions.
The deposition function could use material from a cathodic arc as described in U.S. Pat. NO. 5,279,723--Falabella, et al. As an alternative, the deposition could use material from a magnetron as described in the article by Thornton in the Journal of Vacuum Science and Technology, Vol. 15 (1978), beginning on page 171, or an improved magnetron as described in U.S. Pat. No. 4,588,490--Cuomo, et al. As another alternative, the deposition could use material sputtered from a target that is bombarded by an energetic ion beam from a gridded ion source as described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745.
The energetic ions to perform the ion assisted function may be from a gridded ion source as described in the above article by Kaufman, et al. The energetic ions may also come from a gridless ion source as described in U.S. Pat. No. 4,862,032--Kaufman, et al. Finally, the energetic ions may come from a background plasma, with the energy supplied by a negative bias on the deposition substrate.
While the energetic ions for the ion assisted function may come from different kinds of ion sources, the range of ion energy consistent with low damage to the deposited film is more limited. A variety of ion assisted deposition applications is described in the book Handbook of Ion Beam Processing technology, J. J. Cuomo, S. M. Rossnagel, and H. R. Kaufman, eds., see for example chapters beginning on pages 170, 194, and 373 by E. Kay and S. M. Rossnagel, R. A. Roy and D. S. Yee, and P. J. Martin and R. J. Netterfield, respectively. The maximum energy level for acceptably low damage, either in the film being deposited or the substrate upon which it is deposited, depends on the particular application, but is below about 200-300 eV/ion. In general, then, the energetic ions for the ion assisted function should be below about 200 eV/ion. Please note that this energy is electron-volts per ion, which is separate and distinct from electron-volts per atom.
It should be obvious that various combinations of the above deposition and ion assist devices may be used, examples of which have been incorporated herein by reference. In some cases, there may be differences in operating regimes that require additional apparatus, such as differential pumping to permit a difference in operating pressure for different devices when used simultaneously in the same ion assisted deposition process. Using separate devices as described above for the separate deposition and ion assist functions can result in bulky, complicated, and expensive apparatus.
Attempts have been made to integrate different aspects of the deposition and ion bombardment functions. The abovementioned cathodic arc and magnetron are devices that integrate a target of the material to be deposited with a source of energetic plasma or ions to sputter or otherwise disperse that target material. Thus, to that extent, they are integrated devices compared to the use of a separate gridded source and target as a source of deposition material.
An attempt to integrate all aspects of ion assisted deposition is described by U.S. Pat. No. 4,911,814--Matsuoka, et al. But this attempt has the serious shortcomings of requiring two sputter targets of the same material, a dielectric window that must be protected from sputtered material, and an electromagnet sufficient to generate a magnetic field of about 875 gauss over a substantial volume of the discharge region. These shortcomings result from the use of microwave power to generate the plasma.
There are other problems with prior art. One is that the deposition substrate may include insulating layers which prevent the use of substrate bias for the ion assist function. Another is that excessively high operating pressures are sometimes required in the deposition region, so that material sputtered from the deposition substrate by the energetic ions performing the ion assist function may return to the deposition substrate, thereby increasing the contamination in the thin film being deposited. Yet another is that some devices require large gas flows, resulting in larger, more expensive vacuum pumps.
There can be problems with the cathodic arc described in the aforesaid U.S. Pat. No. 5,279,723--Falabella, et al. or as described in a review article by Sanders in the Journal of Vacuum Science and Technology, Vol. A7 (1989), beginning on page 2339. While the deposition rates can be quite high for cathodic arcs, they consistently eject macroparticles that degrade the quality of the thin films being deposited. The processes responsible for the production of macroparticles are complicated, but they are clearly associated with the cathodic arc and the large electron emission from the target that is required to sustain this arc. While the aforementioned article by Sanders in the Journal of Vacuum Science and Technology teaches filter means for removing these macroparticles, the filter means substantially increases the complexity of the apparatus while decreasing the output of material to be deposited.
There is also prior art that pertains to deposition but differs in important aspects from the invention herein. One example of such prior art is described in U.S. Pat. No. 5,840,167--Kim and differs in using only ions of the sputtered material, as opposed to ion-assist ions of another species, e.g., argon ions. The aforesaid patent also differs in using electrostatic acceleration of only the ionized sputtered material. The accelerated current is very sensitive to acceleration voltage in electrostatic acceleration--it varies as the three-halves power of voltage. This is why the high voltage of 1000 volts was found necessary by Kim to achieve useful ion currents. The deceleration of the ions subsequent to their acceleration serves to reduce ion energies to less damaging values while retaining the high ion current obtained by using the high acceleration voltage. Because of the use of electrostatic acceleration, the deceleration is a necessary step. The use of deceleration increases the complexity of the apparatus, adds one or more power supplies (two in the case of Kim), and is present in all embodiments. Even with the two stages of deceleration of 200 V each, the net ion energy would be 600 eV/ion, which is excessive for applications that require low damage to the deposited film.
Another example of prior art that pertains to deposition but differs in important aspects from the invention herein is U.S. Pat. No. 5,346,600--Nieh, et al., wherein the acceleration of assist ions is obtained by biasing the deposition substrate negative relative to the surrounding vacuum chamber, which requires that the deposition substrate be a conductor. The generation of assist ions also takes place throughout the vacuum chamber in which the apparatus is installed. This means that a high pressure fills that chamber, rather than being limited to the interior of a piece of apparatus (the ion source) within that chamber, which results in the deposition taking place at a high background pressure and therefore incorporating the contaminants of that high pressure. To aid in the ionization throughout the vacuum chamber, the vacuum enclosure has a magnetic field at the walls of the enclosure. (There is no special requirement for the vacuum enclosure used in the present invention.) Consistent with the generation of ions throughout the vacuum chamber, the ions reach the work piece without any significant directed energy as would be associated with an ion beam and therefore acquire energy only as the result of the work piece being biased, i.e., because the target is a conductor.